Abstract
An ultrasonic method using a large bandwidth transducer with a spherical lens and based on acoustic waves separation near the focal region is presented. The aim of this technique is to reduce the investigation size for non destructive mechanical properties evaluation. Compared to traditional acoustic microscopy (acoustic signature) the size of the analysed zone on the sample has been highly reduced. For instance, this technique has been applied on an aluminium sample with an acoustic frequency of 15 MHz. The Rayleigh wave velocity has been measured individually on grains smaller than one millimetre. Such local measurements would have required an acoustic lens working at higher frequency. All the efficiency of our experimental method and numerical signal processing has been proved by conclusive experiments on different materials such as glass, steel, silicon and uranium dioxide at different frequencies. This new method has also been tested at 100 MHz and we have demonstrated that its resolution was similar to performances of higher frequency acoustic microscopy working around 500 MHz. Furthermore our study shows that with this microdefocusing method, it is possible to assess directly from the same acquisition data Rayleigh, longitudinal and transverse velocities and consequently the elastic properties.